This invention relates to a communication system of a code division multiple access (CDMA) type which is capable of carrying out bidirectional communication and which uses a spread spectrum technique and to a base station used in the CDMA communication system which will be simply called a CDMA system hereinunder.
A conventional CDMA system has been proposed in "AN OVERVIEW OF THE APPLICATION OF CODE DIVISION MULTIPLE ACCESS (CDMA) TO DIGITAL CELLULAR SYSTEMS AND PERSONAL CELLULAR NETWORKS" (May 21, 1992) published by QUALCOMM Incorporated and "TIA/EIA INTERIM STANDARD Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System TIA/EIA/IS-95-A (Revision of TIA/EIA/IS-95), May 1995.
In a conventional CDMA system of the type described, a plurality of base stations are spaced apart from one another to define service areas and bidirectionally communicable with a plurality of mobile stations. In other words, bidirectional communication can be made between the base stations and the mobile stations.
In such a CDMA system, the base station forwardly communicates with each mobile station in a service area through a forward or downward channel while each mobile station reversely communicates with the base station or stations through a reverse or backward channel. Herein, it is assumed with the instant specification that the forward and the backward channels are assigned to each mobile station and that the base station has a plurality of transceivers which include transmitter and receiver sections to carry out transmission and reception operations between each transceiver and the mobile station further assigned thereto.
Under the circumstances, an original digital signal, such as an audio or an image data signal, is multiplied by a pseudo random noise or number (PN) code in the transmitter section of the base station to be sent through the forward channel. By such multiplication, the original data signal which has a narrow bandwidth is spread in spectrum into a spread signal of a wide bandwidth. The spread signal is wider in bandwidth than the original data signal. Such a spread signal is subjected to modulation, such as QPSK or the like, and is thereafter transmitted as a forward radio signal through the forward channel to a receiver of each mobile station to be received as a reception signal.
In the receiver of the mobile station, a correlation is calculated between the reception signal and a pseudo random noise (PN) code which is identical with the PN code used in the transmitter. As a result, the reception signal is despread in spectrum to reproduce the original digital data signal.
Herein, it is assumed in the receiver that use is made of a PN code which is different from the PN code used for spreading the spectrum in the transmitter. In this event, the reception signal appears as a wide band noise on despreading and demodulating the reception signal by the use of such a different PN code.
Taking the above into consideration, it will be readily understood that the original digital data signal alone is extracted from the reception signal when the same PN code is used in the transmitter section of the base station and the receiver of the mobile station.
As mentioned before, a plurality of transceivers are included in each base station to define a plurality of channels which are assumed to be determined by a plurality of PN codes. Thus, when the PN codes are assigned to the channels, the original digital data signals can be separated from one another at every one of the channels without any interference, on the condition that mutual correlations among the PN codes assigned to adjacent ones of the channels become sufficiently small. Thus, code division multiple access (CDMA) can be accomplished in the above-mentioned structure and manner.
As mentioned before, it is noted that multiple access is achieved by code division in the CDMA system and that a plurality of base stations share a common frequency band in a forward link, which differs from a frequency division multiple access (FDMA) system and a time division multiple access (TDMA) system. In addition, each mobile station often communicates with a plurality of the base stations in the CDMA system through the forward and the backward channels assigned thereto, as mentioned before.
In the CDMA system, transmission power should be strictly controlled not only at every one of the base stations but also at every one of the transceivers of each base station. This is because interference among the base stations takes place in each mobile station in dependency upon the electric power of each base station. Specifically, when the transmission power is increased in a certain one of the base stations in comparison with the transmission power in the other base stations, such an increase of the transmission power adversely affects radio signals sent from the other base stations and results in interference in each mobile station.
In order to avoid such interference, a capacity of subscribers is determined in the CDMA system on condition that a signal-to-noise (S/N) ratio in each mobile station exceeds a predetermined reference level. The capacity of subscribers becomes small, if a noise is comparatively large. Otherwise, the capacity of subscribers can be increased.
Taking the above into account, the capacity of subscribers is increased by adjusting or controlling the transmission power of each of the base stations in the CDMA system in consideration of communication quality and interference in each mobile station.
At any rate, the transmission power must be controlled in each of the base stations so as to reduce a rate of the interference on the other mobile stations. To this end, a transmission power control unit is included in each of the base stations used in the conventional CDMA system. Specifically, the transmission power on the side of each base station is controlled on the basis of a reception quality factor detected by each mobile station. With this structure, each base station controls the transmission power with reference to the reception quality factor sent from each mobile station so that speech quality is kept appropriate in each mobile station.
However, the transmission power control unit is not always effective to adjust the transmission power in each of the base stations included in the CDMA system. More specifically, it is assumed that first and second base stations are located which have service areas adjacent to each other and that first and second mobile stations run within the first and the second service areas, respectively. Herein, it is further assumed that the first and the second mobile stations communicate with the first and the second base stations through forward links, respectively, but are located at positions influenced by transmission signals sent from the second and the first base stations, respectively.
Under the circumstances, let a reception quality factor in the first mobile station be reduced at a forward channel due to movement of the first mobile station or the like. The first mobile station notifies the first base station of a reduction of the reception quality factor by using a power measurement message. Responsive to the power measurement message, the first base station detects the reduction of the reception quality factor in the first mobile station. After detection, the first base station augments transmission power for the first mobile station so as to improve the reception quality factor of the first mobile station. As a result, the reception quality factor is improved in the first mobile station.
On the other hand, it should be recollected that a transmission signal or a spread signal has a very wide bandwidth in the CDMA system because the spread spectrum CDMA technique is used, as mentioned before. Therefore, augmentation of the transmission power in the first base station brings about an increase of a wide band noise component at the second mobile station which is located at the position influenced by the first base station. This means that a reception quality factor is decreased in the second mobile station because the reception signal to wide band noise ratio becomes small with an increase of the transmission power in the first base station.
A decrease of the reception quality factor in the second mobile station is transmitted by the use of the power measurement message to the second base station. Supplied with the power measurement message, the second base station detects the decrease of the reception quality factor in the second mobile station and augments the transmission power for the second mobile station. Such augmentation of the transmission power serves to improve the reception quality factor in the second mobile station.
This results in a reduction of the reception quality factor in the first mobile station, as already mentioned above.
Thus, the augmentation of the transmission power is alternately repeated in the first and the second base stations until each of the first and the second base stations reaches maximum transmission power. Such a phenomenon may be referred to as a feedback phenomenon. Eventually, the reception quality factors are not improved in the first and the second mobile stations.
Practically, a similar feedback phenomenon also takes place in a single base station when the single base station communicates with a plurality of mobile stations by the use of a plurality of transceivers included in each base station.
From the above, it is readily understood that a reduction of the reception quality factor in a certain mobile station is successively spread or propagated over adjacent ones of the service areas and that the reception quality factor is degraded over a whole of the CDMA system.
Consequently, a reduction takes place as regards the capacity of subscribers and the number of the mobile stations. As a result, it is found out that a radio source which is composed of a set of radio frequencies is not always effectively utilized in the CDMA system.